This invention relates to process controllers and sensors and more particularly to such controllers and sensors suitable for sensing and controlling the concentration of a caustic solution in bottle washing machinery and a sampling device for use therewith.
In the past there have been numerous attempts to automate the control of caustic soda solution concentrations in commercial bottlewashers. These attempts are believed to have had only limited success. For example, there are believed to be no commercially available systems or devices that can achieve caustic solution concentration control at a satisfactory level of accuracy.
The caustic soda solution in a bottlewasher is typically used at strengths of no less than 3% and no higher than 6%. Such a bottlewasher normally includes a number of tanks containing caustic at differing concentrations followed by one or more tanks containing rinse water, which desirably has little or no caustic therein. Thus, the solution strength in any one compartment of the bottlewasher cannot be allowed to vary over the entire range of from 3 to 6%. On the contrary, each bottlewasher tank has rather narrowly defined limits of solution concentration for that tank. Achieving acceptably clean bottles at a given rate of production within the limits of the specific bottlewasher's capacity, requires a predetermined "soaking time" per bottle at specific caustic strengths and specific temperatures.
Presently, the task of maintaining the correct solution strength for each tank is accomplished by periodically manually drawing samples from each compartment or tank for lab analysis. The commonly accepted method of testing is differential titration. A small, carefully measured volume of the sample solution is placed in a glass flask. Phenolphthalein indicator is added to the glass flask, which turns the solution pink. By means of a burette, acid is slowly added to the mixture in the flask until the pink color just disappears. The number of drops of acid used is recorded and used to calculate the per cent caustic.
The problem with differential titration is that the samples drawn from the bottlewasher are very dirty. In fact they are virtually opaque from all the dirt, dissolved labels and the like from the bottles being washed. This fact makes it very difficult to judge the point of color change in differential titration with any high degree of accuracy. In addition, since this titration process is accomplished manually, there is also the ever present possibility of human error. Even assuming that the caustic concentration is measured accurately by this manual process, present systems are adversely affected by the fact that the necessary changes to solution strengths are then made under manual control using the same large diameter fill line and centrifugal pumps which are used to fill the tanks or compartments at the start of the production line. As a result, the amount of caustic that is added to the tank or compartment cannot be accurately determined. In addition, the strength of what is added is generally in doubt because only a single fill line is commonly used for both high and low strength caustic.
The level of solution in the compartments is usually checked manually as well. This checking is necessary since a certain minimum amount of solution is needed in each tank to ensure that all of the bottles are completely submersed. The entire checking process is tedious, labor intensive, and fraught with hazards from constant handling of the highly corrosive material. The process is also slow, and in fact, problems often develop between sampling intervals which are severe enough to cause operational or product quality problems.
Until the present, it has been difficult to automate this testing for several reasons. For one, previous sensor probe designs were prone to virtually constant contamination with scale build-up, sludge, labels, and dirt. Such probes have been extremely unreliable and have required constant maintenance. In addition, previous probes were incapable of discriminating accurately between solutions that vary only slightly in strength. As a result, such probes could not maintain the solution within a close enough tolerance to a desired level to justify their expense, especially when they needed constant cleaning in order to work at all. At least one probe and system, shown in Canadian Pat. No. 913,181, issued Oct. 24, 1972 has been designed to address some of these problems. However, the accuracy, safeguards, and reliability of this particular probe and system could be improved.